The present invention relates to a starting circuit for starting a main circuit operated to enable the stable supply of AC current from an AC power-supply source used for operating an uninterruptible power-supply source to be supplied to an objective load, and to a method of starting the main circuit.
FIG. 5 shows a block diagram of a conventional uninterruptible power-supply source (UPS) disclosed in Japanese Patent Publication (Laid-Open) No. HEISHEI-09-205740. Basically, this conventional UPS system drives a power converter 70 so as to cause a DC current available via an AC power-supply source 1 and another DC current available via a PWM (pulse-width modulation) converter 60 or another DC current directly available from a battery 6 via a DC switch 80 to respectively be converted into AC current consisting of a predetermined constant voltage and a constant frequency, before eventually supplying the constant AC voltage and frequency to external sources. Accordingly, it has been generally practiced to select a DC voltage output from the PWM converter 60 and also to select another DC voltage output from the battery 6 at a level substantially equal to each other. In FIG. 5, an illustration of a charging circuit is omitted.
The individual arms of the PWM converter consist of a diode and a switching element such as an IGBT (insulated gate bipolar transistor), for example, that are connected to the diode via an anti-parallel connection. Normally, an AC voltage is received from an AC power-supply source 1 via an input filter consisting of a main switch 21, a reactor 35, and a capacitor 33, and also via a boosting reactor 31, and the input AC voltage is then rectified into a DC voltage and boosted by the PWM converter 60. Finally, the boosted DC voltage is smoothed by a large-capacity electrolytic capacitor 4 before eventually being fed to a power converter 70. A series circuit comprising a charging switch 22 and a charging resistor 10 is connected to the main switch 21. All operations of the PWM converter 60 and the power converter 70 are individually controlled by a corresponding controlling unit 45. Corresponding controlling units are omitted in FIG. 5.
A startup-sequence controlling circuit 45 is provided so as to control the opening and closing operations of the charging switch 22 and a series switch 80, and also to control the startup of the PWM converter 60. When the main switch 21 and a DC switch 80 remain open and the power converter 70 remains OFF, upon receipt of a startup command and after the elapse of a predetermined delay time, the above controlling circuit 45 starts up operation of the PWM converter 60 to cause the DC voltage of the capacitor 4 to be boosted, and further causes the main switch 21 to be closed after the elapse of a predetermined delay time. By executing the above controlling operations, the capacitor 4 is enabled to implement an initial charging operation via a charging resistor without causing excessive rush current to flow therein.
In the conventional UPS circuit shown in FIG. 5, while the AC power-supply source 1 remains normal, the DC switch 80 remains open, whereas the main switch 21 is closed. AC voltage is received from the AC power-supply source 1 via the main switch 21 and an input filter. Next, the input AC voltage is converted into DC voltage by the PWM converter 60, and then smoothed by the capacitor 4. Then, the smoothed DC voltage is converted into AC current by the power converter 70 and, finally, the AC current is supplied to a load (not shown). In the event of the AC power-supply source 1 incurring a service interruption, as a means of avoiding instantaneous service interruption under a load by feeding a voltage stored in the capacitor 4, the controlling circuit 45 causes the DC switch 80 to be closed so as to ensure that an AC voltage will be transmitted to the power converter 70, thus achieving a system for effectively preventing instantaneous service interruption.
In the above circuit arrangement, in order to normally charge the capacitor 4 without causing an excessive rush current to flow therein when the charging operation is started, it is required to install a charging switch 22 and a charging resistor 10 in parallel with the main switch 21 disposed between the AC power-supply source 1 and an AC input filter of the PWM converter 60. It is further required to provide another startup sequence controlling circuit 45 for controlling those additional switches. This in turn necessitates the provision of an increased number of component parts, thus obstructing cost reduction and downsizing of the UPS unit. Further, as the level of the DC voltage stored in the battery 6 is substantially equal to that of the PWM converter 60, in the case of a conventional UPS system with a modest output capacity, the number of serial connections of the battery inevitably increases, thus adversely affecting the cost reduction and downsizing of the UPS system.
To solve the above problems, as cited in Japanese Patent Publication (Laid-Open) No. 2002-199620, there is a method of installing a pressure-adjustable chopper between a smoothing capacitor and a battery. According to this method, the pressure-adjustable chopper is operated so as to lower the voltage level in order to implement charging of the battery when the AC power-supply source remains normal. Conversely, when the AC power-supply source incurs a service interruption, the above chopper is operated so as to boost the voltage level, thereby boosting the voltage level of the battery to the voltage level concurrently output from the PWM converter 60. Accordingly, it is possible to select a low voltage for charging of the above battery. The previous art cited in Japanese Patent Publication (Laid-Open) No. 2002-199620 has also introduced the arrangement shown in FIG. 4 for implementation of the initial charging of the capacitor 4 when operation of the conventional UPS system is started.
In light of the above-described problems, it would be desirable to provide a power-converting apparatus featuring the ability to achieve initial charging of a smoothing capacitor generating a minimum rush current by obviating the need for additional series connections in a battery, and also obviating the need for an initial charging circuit consisting of a charging resistor and switch in the capacitor.